Various types of coated cutting tools have been proposed. For example, the below-listed patent documents 1 to 6 have disclosed tools that have a coating on their substrate. An example of the structure of the coating is composed of an inner layer made of a Ti-based material, an outer layer made of Al2O3, and an outermost layer made of a Ti-based material. However, the bonding strength between the inner and outer layers is insufficient in most cases. To solve this problem, various bonding layers have been proposed.    (Patent document 1): published Japanese patent application Tokukoushou 57-42152    (Patent document 2): published Japanese patent application Tokukoushou 58-67858    (Patent document 3): published Japanese patent application Tokukoushou 59-44385    (Patent document 4): published Japanese patent application Tokukoushou 60-37189    (Patent document 5): published Japanese patent application Tokukouhei 1-12835    (Patent document 6): published Japanese patent application Tokukaihei 5-8103
On the other hand, the cutting load applied to the coating has extremely increased in recent years because the complicated shape of workpieces increases the number of working contacts and difficult-to-machine materials increase the amount of adhesion to the tool. More specifically, the increase in the amount of adhesion increases cutting resistance. As a result, film spalling or film wearing occurs abruptly, decreasing the tool life.